In general, an image sensor is a semiconductor device for converting an optical image into electrical signals, and may be classified as a Charge Coupled Device (CCD) image sensor or a Complementary Metal Oxide Semiconductor (CMOS) image sensor.
A CMOS image sensor includes a photodiode for detecting emitted light, and a logic circuit for converting the detected light into electrical signals and generating data from the electrical signals. As the amount of light received in the photodiode increases, the photo sensitivity of the image sensor becomes better.
In order to improve the photo sensitivity of a CMOS image sensor, various methods have been employed. One method increases a fill factor of a photodiode with respect to the entire area of an image sensor. Another method incorporates a light condensing technology, which changes the path of light incident into an area not having a photodiode, thereby causing light to be concentrated to the photodiode.
One example of the light condensing technology is to form a microlens. In particular, a convex microlens made from a material having good light transmittance is formed on the upper portion of the photodiode, and refracts the path of incident light, thereby causing more light to be incident into the photodiode.
In such a case, light parallel with the optical axis of the microlens is refracted by the microlens, and the focus of the light is formed at a certain position on the optical axis.
In general, an image sensor includes a photodiode, an interlayer dielectric, a color filter, and a microlens.
The interlayer dielectric is formed on a semiconductor substrate having a plurality of photodiodes formed therein, and red, green, and blue (RGB) color filter layers are formed on the interlayer dielectric so as to correspond to the photodiodes.
A planarization layer is formed on the color filter layer in order to planarize a surface of the color filter layers, and microlenses are formed on the planarization layer corresponding to the color filter layers.
In operation, the photodiode converts detected light into electrical signals, the interlayer dielectric insulates metal wirings, the color filter expresses the three primary colors RGB of light, and the microlens causes light to be concentrated to the photodiode.
Hereinafter, a conventional process for forming a microlens will be described with reference to the accompanying drawings.
FIGS. 1a to 1d are sectional views illustrating a process for forming a microlens according to the prior art.
As illustrated in FIG. 1a, a microlens layer 52 is formed on a semiconductor substrate 10, having a plurality of photodiodes 40, on which an interlayer dielectric 20, a color filter layer 30 and a planarization layer 25 have been formed.
Referring to FIG. 1b, the microlens layer 52 is patterned so as to correspond to the photodiodes 40, respectively.
Referring to FIG. 1c, the semiconductor substrate 10 is placed on a hot plate 60, and then is heated to reflow the microlens layer 52 to form the microlenses 50 shown in FIG. 1d. 
Referring to FIG. 1d, the microlenses 50 are non-uniformly formed as illustrated by references A and B.
When the semiconductor substrate 10 including the microlens layer 52 is located on the hot plate 60 and directly heated, the heat transfer coefficient, or thermal capacity, locally differs depending on the heat transfer paths below the microlens layer 52. In addition, the heat transfer amount may locally differ by a step difference. Therefore, non-uniform thermal flowing occurs. Accordingly, non-uniform microlenses 50 are formed.
When the microlenses 50 attach to each other and are unequally spaced apart as illustrated by reference A of FIG. 1d, or the curved surfaces of the microlenses 50 are distorted as illustrated by reference B of FIG. 1d, the non-uniformity may affect adjacent pixels, and concentration efficiency may deteriorate.